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OPEX & Quad Witching

Updated Jun 24, 2026 at 8:22pm

Research Draft High 1,265 words

OPEX ("options expiration") is the calendar event when listed options stop trading and settle. In US equities the standard monthly OPEX is the third Friday of every month, when monthly stock and index options expire. Four times a year — the third Friday of March, June, September, and December — that monthly expiration coincides with the quarterly expiration of stock-index futures and index-futures options, producing the high-volume session traders call triple witching (historically "quadruple witching" when single-stock futures also expired; those products are no longer a regular part of US markets, so "triple witching" is now the accurate term). The core tension of the topic: expiration is a real, mechanical liquidity and hedging event, but the popular narrative around it — that prices get "pinned" or that the day is reliably volatile and tradable — is far weaker and more contested than the folklore suggests.

How it's formed

Three things converge on an expiration date:

1. Contract settlement. Open interest in expiring contracts must be closed, rolled to the next cycle, exercised, or allowed to expire. On a typical monthly OPEX roughly $1–2 trillion notional of options expire; on quarterly witching that can reach $4–5 trillion (practitioner estimates, e.g. gexmetrix, iPresage — treat as order-of-magnitude, not precise). 2. Settlement mechanics differ AM vs PM. Standard monthly/quarterly index options (e.g. SPX) are AM-settled using the Special Opening Quotation (SOQ) — a synthetic index value built from each component's opening trade on expiration Friday (Cboe). Equity options and most weeklys are PM-settled at the 4:00 PM close. This is why witching-day flow concentrates at the open for index products and at the close for single stocks and futures rolls. 3. Dealer gamma decay. As expiration nears, gamma on near-the-money options spikes. Market makers who are net-long or net-short those options must rebalance delta hedges aggressively into the final hours. After the contracts expire, that hedging demand vanishes, removing a source of intraday stabilization (or destabilization).

How it's used in practice

Practitioners watch the expiration cycle for three things:

  • Liquidity / execution windows. Witching days are routinely among the highest-volume sessions of the year, with much of it in the closing auction and the SOQ open. Index-rebalance flows often coincide. Large orders get better fills; spreads can also gap. This is the least controversial, most durable use.
  • Open-interest "magnets." Strikes carrying very large open interest (a "max-pain" or high-gamma strike) are watched as potential resting levels into a PM-settled expiration. Dealer desks long gamma at a strike hedge against movement (buy dips, sell rips), which can compress range toward that strike — the proposed "pinning" mechanism.
  • Post-OPEX regime shift. A common framing: the week before monthly OPEX tends to be calmer (the "gamma cushion" is maximal), and the week after quarterly expiration can see higher realized volatility as the expiring hedge book — and its stabilizing flow — rolls off. This is a positioning heuristic, not a guarantee.

Adoption, debate & evidence

The evidence splits sharply by era and instrument, and conflating them is the single most common error.

  • Pinning — strong in 2000s single-stock options. Ni, Pearson & Poteshman (2005, Journal of Financial Economics; sample 1996–2002) found robust evidence that optionable US stocks cluster at strike prices on expiration dates, attributing it to delta-hedge rebalancing (per Avellaneda & Lipkin) and option-writer hedging. They estimated expiration-date returns were altered by at least ~16.5 bp on average, which they translate into market-cap shifts on the order of ~$9 billion per expiration. This is the canonical, peer-reviewed support for pinning. A separate study — Golez & Jackwerth (2012, JFE) — found pinning in the S&P 500 futures over 1998–2009 amounting to a notional move of at least ~$240 million per expiration day, so an index-level pinning effect was documented for that earlier window (note this is a distinct paper, not a follow-up by NPP).
  • Pinning — weakened/reversed in the most recent index/0DTE era. Pinning was documented for monthly equity options but was not significant for weekly options. More recent work spanning the 2016–2025 weekly/0DTE proliferation era reports no evidence of pinning in S&P 500 expiration dynamics; high near-expiry ATM open interest was associated with wider, not narrower, ranges — consistent with a gamma-amplification mechanism rather than pinning. So while index-level pinning was measurable in the pre-2010 futures window (Golez–Jackwerth, above), the contemporary index market may behave oppositely to both that and the 2000s single-stock result.
  • 0DTE has diluted the monthly signal. Same-day options have become the majority of SPX option volume — Cboe reported SPX 0DTE share running around the high-50s to low-60s percent in 2025 (e.g. a record ~62% in August 2025), up from roughly a fifth of total US listed-options volume. Because 0DTE gamma resets every afternoon, the monthly third-Friday concentration matters less than it did pre-2022; each session "starts fresh."
  • Witching returns/volatility — conflicting. Studies on witching-day volume agree it spikes (commonly cited as +50% or more vs normal days). But effects on returns and volatility are genuinely mixed: some event studies find exploitable abnormal returns (e.g. on Nasdaq, inconsistent with EMH), others detect no significant price/volatility effect attributable to expiration itself. Any effect is consistently described as short-lived.

Bottom line: volume effects are real; pinning is real for 2000s single-stock options but unreliable-to-reversed for modern index/0DTE; tradable return edge is unproven and contested.

Strengths & limitations

When it's useful: as a liquidity and event-timing awareness tool — knowing that the third Friday brings a large auction, AM index settlement, index rebalances, and a post-expiration change in dealer positioning. As context, it improves execution and risk timing.

When it fails: as a directional signal. "Buy/sell the witch" and naive max-pain targeting have no robust, out-of-sample edge, and the pinning literature does not transfer cleanly to today's index/0DTE structure. The #1 misuse: citing the Ni-Pearson-Poteshman pinning result (single stocks, 2000s) to justify trading SPX/SPY index pins today — the modern evidence points the other way (amplification). A second misuse is treating practitioner notional figures ($X trillion expiring) as precise — they're estimates.

Sources

Dispute flags: (1) Pinning is era/instrument-dependent — robust for 2000s single-stock options, contested-to-reversed for modern index/0DTE. (2) Witching return/volatility effects are conflicting across studies; no established tradable directional edge. (3) Notional dollar figures are practitioner estimates, not exact.